Hydration Effect on Equations of State for Minerals in the Peridotite System:Implication for Geotherms in the Mantle

Water in the deep Earth’s interior has important and profound impacts on the geodynamical properties at high-temperature(T)and high-pressure(P)conditions.A series of dense hydrous Mg-silicate(DHMS)phases are generated from dehydration of serpentines in subduction slabs below the lithosphere,including phase A,chondrodite,clinohumite,phase E,superhydrous phase B and phase D.On the other hand,olivine and its high-P polymorphs of wadsleyite and ringwoodite are dominant nominally anhydrous minerals(NAMs)in the upper mantle and transition zone,which could contain significant amount of water in the forms of hydroxyl group(OH-)defects.The water solubilities in wadsleyite and ringwoodite are up to about 3 weight percent(wt.%),making the transition zone a most important layer for water storage in the mantle.Hydration can significantly affect the pressure-volumetemperature equations of state(P-V-T EOSs)for the DHMS and NAM phases,including the thermal expansivities and isothermal bulk moduli.In this work,we collected the reported datasets for the DHMS and NAM phases,and reconstruct internally consistent EOSs.Next,we further evaluated the thermodynamic Grüneisen parameters,which are fundamental for constraining the temperature distribution in an isentropic process,such as mantle convection.The adiabatic temperature profiles are computed for these minerals in the geological settings of normal mantle and subduction zone,and our calculation indicates that temperature is the dominant factor in determining the gradient of a geotherm,rather than the mineralogical composition.

Crystal Structure,Thermal Expansivity and High-Temperature Vibrational Spectra on Natural Hydrous Rutile

A natural rutle sample was measured by in situ high-temperature X-ray diffraction(XRD)patterns,as well as Raman and Fourier transform infrared(FTIR).Crystal structure is refined on the sample with 1.4 mol.%Fe and 510±120 ppmw.H2 O.The unit-cell and Ti O6 octahedral volumes are expanded by 0.7%–0.8%for Fe3+incorporation,as compared with the reported Ti-pure samples.The volumetric thermal expansion coefficient(α,K-1)could be approximated as a linear function of T(K):4.95(3)×10-9×T+21.54(5)×10-6,with the averaged valueα0=30.48(5)×10-6 K-1,in the temperature range of 300–1500 K.The internal Ti-O stretching(A1 g and B2 g)and O-Ti-O bending(Eg)modes show‘red shift',whereas the multi-phonon process exhibits‘blue shift'at elevated temperature.The rotational mode(B1 g)for Ti O6 octahedra is nearly insensitive to temperature variations.The OH-stretching bands at 3279 and 3297 cm-1 are measured by high-temperature spectroscopy experiments.Both the IR-active and Raman-active OH-stretching modes shift to lower frequencies at higher temperature,with the signal intensities decreasing.And after quenching,we expect about 43%dehydration around 873 K,and 85%dehydration at 1273 K for this hydrous sample.